Tetragrammaton n 1: four Hebrew letters usually transliterated as YHWH (Yahweh) or JHVH (Jehovah) signifying the Hebrew name for God which the Jews regarded as too holy to pronounce
Tetragrammaton \Tet`ra*gram"ma*ton\, n. [NL., fr. Gr. ?; te`tra- (see {Tetra-}) + ? a letter.] The mystic number four, which was often symbolized to represent the Deity, whose name was expressed by four letters among some ancient nations; as, the Hebrew JeHoVaH, Greek qeo`s, Latin deus, etc. [1913 Webster]